Amazed to hear this! Yep that’s actually possible, you would need only one relay and a handful of diodes to make a simplest one relay automatic battery charger circuit.
How it Works
The idea struck me while trying to design the easiest possible battery charger circuit for one my clients.
The concept is simple; just raise the operating or triggering voltage of the relay up to the optimal battery charging threshold voltage by dropping the required amount of supply voltage to the relay coil, with the help of series diodes.The idea may be understood from the following points:
Take an ordinary relay, measure its triggering voltage by carefully applying a variable voltage across its coil.Now suppose the triggering voltage of the particular relay was about 9 volts, and also assume you want to raise its voltage to 14 volts, which may be your 12 volt battery’s charging threshold voltage.
We know that a 1N4007 diode drops about 0.6 volts across it, so if we add sufficient number of diodes in series with the relay coil would hopefully pull its tripping or triggering voltage to about 14 volts.
That means, 14 – 9 = 5, we’ll require 5/0.5 = 10 diodes in series to achieve this rise in the triggering voltage of the relay.That’s pretty simple and interesting isn’t it?The rest may be done with the help of the shown diagram…..your simplest single relay automatic battery charger is ready.